Conversion coating solution for metallic surfaces

ABSTRACT

A CHROMATE CONVERSION COATING COMPOSITION USEFUL FOR APPLYING TO METALLIC SURFACES COMPRISING AN EFFECTIVE CORROSION REDUCING AMOUNT OF A WATER-SOLUBLE PARTIALLY ESTERIFIED ALKYL PHOSPHATE.

United States Patent Int. Cl. C23f 7/26 US. Cl. 148-6.16 13 Claims ABSTRACT OF THE DISCLOSURE A chromate conversion coating composition useful for applying to metallic surfaces comprising an elfective corrosion reducing amount of a water-soluble partially esterified alkyl phosphate.

This invention relates to a conversion coating solution to form anti-corrosive and adhesive coatings on the surfaces of metals such as steel plates, zinc galvanized steel plates and aluminum or aluminum alloy plates, and especially relates to a conversion coating solution to form, the so-called, chromium-chromate coating.

The coating solution of the invention is chromate coating solution which contains, as its principal components, tri-valent and/or hexa-valent chromium ions and water soluble partially esterified alkyl phosphates. The characteristic feature of the invention is that said solution contains water soluble partially esterified alkyl phosphate, as one of the principal components.

Said water soluble partially esterified alkyl phosphates as being employed in the present invention are generally an equi-molar mixture of the monoester and the diester of orthophosphoric acid having the following formulae:

in which R is an organic radical. It is to be appreciated however, that greater and lesser ratios may also be used than equi-molar amounts.

By applying the above described treatment solution on the surfaces of metals by means of the coating treatments such as immersing the metals, transparent and colorless chromium-chromate coating with superior qualities such as anti-corresiveness and adhesiveness can be formed, where the chromium content of the coating is from 2 to 20 mg./m.

The conversion coating solution and the coating method for metallic surfaces with the principal object of improving the anti-corrosiveness of the coating are disclosed in US. Pat. No. 3,284,319, in which hexa-valent chromium ion (chromic anhydride) and water-insoluble trialkyl phosphates (tributyl phosphate etc.) are used as the main ingredients thereof. In Canadian Pat. No. 836,- 643, an improvement of the above US. patent with regard ot the adhesiveness is disclosed. However, in these patented inventions, water-insoluble trialkyl phosphate, i.e., completely esterified water-insoluble phosphoric ester is used. Accordingly, in the former invention, the adhesiveness of the coating is not good, and in the latter invention, troublesome treatments such as preheatings of the metals to be treated and of the treating solution, pH controlling of the treating solution and the addition of the emulsifying agents are necessary.

Chromate treating solutions usually contain hexa-valent and/or tri-valent chromium compounds as their principal components, as well as one or more additional components, such as, sulfuric acid, phosphoric acid, fluorine compounds, organic acids and organic high polymers. The anti-corrosiveness of the coating on the metal with the use of these treatment solutions, increases in proportion to the thickness of the coating, in which the color of the coating also varies from clear transparent to nudesirable brownish. The relations between the Weights of coatings (chromium contents) and changes of the colors of coatings are as follows:

Weight of coating: Color 15 mg/m. or less Clear colorless. 30-50 mg./m. Pale yellow. mg./m. or more Yellow or golden yellow.

The appearance of the coating on the treated metal with chromate treatment is however desirable to be colorless and transparent keeping the original appearance of the metal surface, and the opaque yellowish coating is not desirable. In other words, the chromate coating is desirable to be thin in order to be colorless and transparent, and in addition that it be anti-corrosive. However, by the use of the ordinarily known treatment solutions or methods, such desirable coatings could not be formed.

The above desirable results can be obtained using a coating solution containing, as the principal components thereof, tri-valent and/or hexa-valent chromium ions and water-soluble partially esterified alkyl phosphates. The aforementioned defects in the use of prior solutions can be overcome by the use of the coating solution of the present invention when applied to iron sheets, zinc galvanized steel sheets and aluminum or aluminum alloy sheets to form the desired coatings.

In accordance with the invention, a superior chromiumchromate coating can be formed on the surfaces of metals, which is thin, colorless and transparent (chromium content: 2 to 20 mg./m. which coating has good anti-corrosiveness and good adhesiveness. The composition of the conversion coating of the invention consists of chromiumchromate as the principal ingredient and a small amount of chromium phosphate, and this chromium phosphate provides the improvements of the properties of the conversion coating of the invention.

The characteristic components of the water-soluble partially esterified alkyl phosphates in the invention have the aforementioned chemical structures, that is, an equi-molar mixture of orthophosphoric monoester and diester, in which in accordance with the results of the experiments, said R, i.e., organic radical may be preferably selected from the following groups in order to obtain an adsorption onto the metallic surfaces, while still obtaining anticorrosiveness of the treated metals and stability of the compounds in the treating solution; the R radical preferably has from 6 to 35 carbon atoms in the molecule and may be selected from the group consisting of alcohols such as long chain fatty alcohols containing 6 to 20 carbon atoms, glycol ethers, such as, long chain fatty alcohol glycol ethers, phenolic glycol ethers or alkylated phenolic glycol ethers.

Some specific examples of R may be as follows:

(1) Lauryl alcohol.

(2) A mixture of hexyl alcohol, octyl alcohol and decyl alcohol.

(3) A mixture of decyl alcohol, lauryl alcohol and myristyl alcohol.

(4) A mixture of cetyl alcohol and stearyl alcohol.

(5) Tributyl phenol tetraglycol ether.

(6) Nonyl phenol tetraglycol ether.

(7) Stearyl alcohol octaglycol ether.

The concentration of the above described water-soluble partially esterified alkyl phosphate in the coating solution of the invention, is in the range of from 0.001 to 20 g./l. and those of chromium ions are in the ranges of from 0.1 to 5 g./l. as tri-valent chromium ion and from 0.1 to 50 g./l. as hexa-valent chromium ion. The aforementioned effects of the invention can be expected with the concentrations in the above-mentioned ranges. Further, the treatment solution of the invention having the above-mentioned concentration may have a pH within the range of from 2.0 to 9.0.

An addition to the above-mentioned components, the treatment solution of the invention may be added with the ordinary additives used in the prior chromate coating solutions such as phosphoric acid or the salts thereof, fluorine compounds, boron compounds or metal ions such as calcium, zinc, aluminum and titanium, in accordance with the kinds of the metals to be treated in order to improve the properties of coating solutions or those of the coating films.

The method of the treatment with the solution of the invention is the same as that of the ordinary chrom-ate coating process. The solution of the invention can be applied on the clean and active metallic surfaces by means of immersion coating, spraying or roller coating, if necessary after squeezing with a roller, the coated layer is subjected to self drying or forced drying.

Thus prepared chromium-chromate coating of the invention is a thin and clear coating film containing a small amount of chromium phosphate, i.e., 2 mg./m. to 20 mg../m. as chromium, and is superior in its anti-corrosiveness and adhesiveness of the film as compared with the coating obtained from the ordinary treatment solution not containing water-soluble partially esterified alkyl phosphate.

In the following, some examples of the present invention are shown.

EXAMPLE 1 Into 800 ml. of water, 7.4 g. of chromic anhydride, 0.2 g. of fluorosilicic acid and 0.3 g. of methyl alcohol were added and boiled to form 1 g. of tri-valent chromium by completely oxidizing the methyl alcohol and partially reducing the chromic anhydride. This solution was added with 0.11 g. of partially esterified orthophosphoric acid having an organic radical of stearyl alcohol octaglycol ether, and then it was diluted with l 1. of water. The pH of this solution was 2.0.

The above described treatment solution was heated up to 40-50 C., and a clean and active steel plate was immersed into the solution for seconds; then the plate was squeezed with a roller and was subjected to hot blast drying for 30 seconds at a temperature of 80100 C. The obtained coating film was colorless and transparent and adhered tightly to the plate. The chromium content in the film was 10-13 mg/mfi.

Further for comparison, a treatment solution was prepared by using the same amounts of the abovementioned components except the partially esterified orthophosphoric acid, to treat another test plate of the same kind under the same conditions. After the drying, this comparative test plate was also provided with colorless and transparent coating ttilm, and the content of chromium was almost the same as that of the former test plate of the invention.

The tests of the anti-corrosiveness in accordance with 118 (Japanese industrial Standard) Z-2371, salt-spray tests were carried out with regard to the above two test plates and to another not treated plate, in which the areas (percent) of the rust formed were compared. The results of the tests are shown in the following table, in which the superiority of the plates treated in accordance with the invention are shown.

Rusted area in salt-spray tests (percent) 4 EXAMPLE 2 Into 500 ml .of water, 1.5 g. of ammonium bichromate 0.5 g. of glycine, 0.1 g. of basic chromium carbonate and 0.1 g. of partially esterified orthophosphoric acid having nonyl phenol tetraglycol ether as organic radical were added, stirred well, and 1 liter of water was added to the obtained solution. The pH of this treatment solution was 5.5.

This solution was then heated up to 4050 C., and aluminum alloy plate (2S) was sprayed with this solution, then squeezed with a roller, and after that the plate was dried with hot blast at a temperature of 80l00 C. for 30 seconds.

The obtained coating was clear and transparent, and adhered tightly to the surface of said aluminum plate. The chromium content in the coating was only 5-6 mg./ m however the result of the salt-spray test was excellent, that is, in the salt-spray test, the anti-corrosiveness of the ordinary coating having about the same thickness was 10-20 hours, while that of the coating of the invention was 100 hours or more.

EXAMPLE 3 Into 500 ml. of water, 4.2 g. of chromic anhydride, 0.2 g. of zinc fluoride, and 0.15 g. of methyl alcohol were added with stirring to produce 0.5 g. of tri-valent chromium ion by reducing a part of the chromic anhydride. Further, I g. of glycine, 0.5 g. of water and 0.05 g. of partially esterified orthophosphoric acid having stcaryl alcohol octaglycol ether as the organic radical was added, and then further 1 l. of water was added thereto. The pH of the obtained solution was 3.0.

The above solution was kept at a temperature of 60- C. and applied on the surface of a clean and active zinc galvanized steel plate by means of spraying, then squeezed with a roller. Thereafter, the plate was dried with hot blast at a temperature of about -100 C. for 30 seconds.

The coating formed was colorless and transparent and was adhered tightly to the surface of the zinc galvanized plate, the chromium content of which was about 6 mg./ m. The anti-corrosiveness of this plate was 24 hours in the salt-spray test, which was far superior to the anticorrosiveness of the similar ordinarily known coatings (about 2 hours).

Then, another zinc galvanized steel plate was treated with a similar coating solution, in which said solution was prepared by mixing the same contents as described in the above except that the partially esterified orthophosphoric acid was excluded. The coated plate was then subjected to the salt-spray test and wasobserved that the plate gathered rust within 6-10 hours. Therefore, it will be understood that the obvious difference of the anti-corrosiveness depends upon the existence of the partially esterified orthophosphoric acid in the coating solution.

A zinc galvanized steel plate which was treated with the abovementioned coating solution was applied with alkyd melamine paint and it was subjected to baking finish at 130 C. and for 20 minutes. The coated plate was then tested for adhesiveness by the cross hatch test with regard to the coating film (thickness: 27-30 microns). The results showed that more than of the paint film adhered to the plate.

What is claimed is:

1. An aqueous conversion coating composition useful for applying to metallic surfaces comprising trivalent chromium ions in an amount ranging from about 0.1 to 5 g./l., hexavalent chromium ions in an amount ranging from about 0.1 to 50 g./l. and a water soluble partially esterified alkyl phosphate wherein the phosphate is present in an amount ranging from about 0.001 to 20 g./l.

2. The composition of claim 1 wherein the phosphate is a mixture of a monoester and diester of orthophosphoric acid.

3. The composition of claim 2 wherein said monoester has the formula (O)P(0R) (OH) 2 and said diester has the formula -(O)P(OR) (OH) wherein R is an organic radical having from 6 to 35 carbon atoms.

4. The composition of claim 3 wherein R is derived from a compound independently selected from the group consisting of an alcohol and a glycol ether.

5. The composition of claim 4 wherein the alcohol is a long chain fatty alcohol and the glycol ether is independently selected from the group consisting of a long chain fatty alcohol glycol ether, a phenolic glycol ether and an alkylated phenolic glycol ether.

6. The composition of claim 1 wherein the pH ranges from about 2.0 to about 9.0.

7. A method of treating metallic surfaces with an aqueous chromate conversion coating comprising contacting the metallic surface with the composition of claim 1 for a period of time sufiicient to form an adhesive transparent chromium containing coating.

8. The method of claim 7 wherein the amount of chromium on the metallic surface ranges from about 2 to 20 mg./m.

9. The method of claim 7 wherein the phoshpate is a mixture of the monoester and diester of orthophosphoric acid.

10. The method of claim 9 wherein said monoester has the formula (O)P(OR) (0H),, and said diester has the 6 formula (0)P(OR) (OH) wherein R is an organic radical having from 6 to carbon atoms.

11. The method of claim 10 wherein R is derived from a compound independently selected from the group consisting of an alcohol and a glycol ether.

12. The method of claim 11 wherein the alcohol is a long chain fatty alcohol and the glycol ether is independently selected from the group consisting of a long chain fatty alcohol glycol ether, a phenolic glycol ether and an alkylated phenolic glycol ether.

13. The method of claim 7 wherein the pH ranges from about 2.0 to about 9.0.

References Cited UNITED STATES PATENTS 2,080,299 5/ 19'37 Benning et 'al. 148 6.15 R 2,224,695 12/1940 Prutton 1486.15 R 2,840,498 6/1958 Logue et al. 1486.l5 Z 3,276,916 10/1966 Wurstner 148--6.16 3,284,319 11/1966 Hill 148--6.16 X 3,445,298 5/1969 Rucoba et al 1486.16

RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 148-62 

